1. Field of the Invention
The invention relates to antifriction systems for reducing frictional stress and wear between well string elements and well conduits containing such strings.
2. Description of the Prior Art
Directional drilling is widely utilized today for enabling a number of wells to be drilled and serviced from a single surface station. The use of directional drilling is of particular importance in the exploitation of offshore oil fields because the manufacture, installation and maintenance of each offshore platform are tremendously expensive, and directional drilling enables a number of separate wells to be drilled and serviced from each platform.
However, the number of wells that can be drilled and serviced from a single surface site such as an offshore platform is limited by the amount of lateral displacement that can be achieved in the directional drilling, which in turn is limited by the large amounts of both rotational and longitudinal friction which are generated between the drill string and casing in deep, widely displaced wells. During the directional drilling of deep, widely displaced wells numerous bends are usually introduced by variations of both azimuth and inclination or drift angle endeavoring to reach a designated target zone. These bends cause the well bore to be somewhat like a roller coaster, which for a deep well, as for example 10,000 to 15,000 feet in depth, results in a great deal of friction between the relatively stiff drill string and the casing. Because of such friction rotating tool joints of the string can cause severe damage to the casing, and a combination of rotating and lineal pulling frictional stressing can cause very high surface power requirements, and even cause a failure of the drill string at depth. Because of similarly high frictional engagement between a fishing string and the casing wall, the part of the string remaining in the well may not be retrievable, and the costly well may be lost. An example of such a failure recently occurred in a well in the Torre platform in the Ekofisk field in the North Sea offshore from Norway. In that well a 95/8 inch O.D. casing had been set to a depth of approximately 13,000 feet with a lateral displacement of approximately 7,000 feet, and an attempt was being made to drill an 81/2 inch hole into a target production zone at about a 14,000 foot depth. The combination of rotational and lineal friction got so great that it exceeded the yield strength of the drill pipe, so that the drill pipe snapped at a depth of approximately 12,000 feet. The frictional stress would similarly have been too great on any state-of-the-art fishing equipment, so that the well was lost.
Under the current state of the art the amount of lateral displacement is limited to about 7,000 to 8,000 feet for a well depth of about 15,000 feet. However, currently there is large demand for directional wells having greater deflection angles and displacements, and greater depths, particularly to reduce the number of offshore platforms that are required in ocean floor drilling and production.
There have been numerous attempts in the prior art to reduce frictional wear and stress between well strings and well conduits containing such strings. Many of these attempts have utilized rollable bearing means such as ball bearings, roller bearings and wheels. However, none of these prior art antifriction systems has been particularly effective in reducing both lineal and rotational friction, while at the same time protecting a surrounding casing from damage by the string. Accordingly, under the present state of the art rollable bearing devices are not even used in drill strings, and protective rubber collars are clamped around the drill pipe just above every other tool joint (i.e., every 60 feet) to prevent the tool joints from frictionally damaging the casing. However, these protective rubber collars actually increase both the lineal and rotational frictional load between the string and the casing, thereby reducing the depth and lateral displacement achievable in drilling a well.
A protective device similar to the aforesaid rubber collar protector is disclosed in Rosser U.S. Pat. No. 3,948,575, and constitutes a resin collar that is molded around a drill pipe, with an outer surface that is slick for reduced friction.
Prior art rollable bearing devices which had direct contact between rollable bearing means and the casing wall were predominantly of the roller bearing type for good load distribution. While such rollable bearing antifriction devices were good for reducing rotational friction, they did not serve to materially reduce lineal friction. Examples of prior U.S. patents disclosing such direct contact roller bearing antifriction devices are Nos. 1,701,885 to Hardesty, 1,890,530 to Santiago, 1,923,328 to Reed, 1,905,158 to Craig et al, and 2,886,288 to Gehrke.
Most prior art ball bearing type antifriction devices used in connection with drill strings embodied ball bearingsupported outer collars which engaged the casing, and did not materially reduce lineal friction. Examples of prior U.S. patents of this type are Nos. 790,330 to Terry, 1,651,088 to Fentress, 1,756,195 to Holdaway, and 1,877,395 to Goeser.
Prior art disclosures of externally exposed antifriction balls in well devices utilized individually mounted balls that were not mounted for free flowing movement in an endless or continuous race. While such ball type antifriction devices did tend to reduce both rotational and lineal friction, the individual ball mounting still produced considerable friction both rotationally and lineally, and the individually mounted balls, being widely spaced apart, produced high pressure point contacts likely to damage casing or other surrounding tubing. Examples of prior U.S. patents disclosing structures of this type are Nos. 1,517,026 to Smith, where the individual balls were mounted in boxes joining sucker rods; 1,801,294 to Sutton, where the individual balls were positioned in tubular guide bearings on sucker rods; and 1,699,087 to Woodmansee, where the individual balls were mounted on a collar either on a drill pipe or on a sucker rod. Similar, noncirculating, and hence relatively high friction, ball bearing type devices capable of lineal antifriction action but inwardly exposed from a collar to a sucker rod were disclosed U.S. Pat. Nos. 1,566,451 to Vaughn and 2,127,796 to Willis; however, this type of collar was limited in its ball bearing engagement to a reciprocating type of device like a sucker rod, and had only a skid type of device for engaging the outer casing. Another type of lineal antifriction device was the longitudinally directed wheel, this being disclosed in U.S. Pat. Nos. 1,281,756 to Black and 1,827,835 also to Black; but these were only usable with sucker rods, and could not be used in connection with drill strings because the wheels extended through the diameters of the strings to both sides thereof.